Method of making permeselective membranes



July 14, 1959 B. G. HARPER ET AL METHOD OF' MAKING PERMESELECTIVE MEMBRANES Filed March l, 1956 2 Sheets-Sheetl l sar/ace o ermembrane Foram/hu/a ermese/ec/ive memb'rane aV/'ng Foram/na/ou ,ooms/'6 mese/ec /ve Pressure O/'f/eren//a cm Hy.

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HTTORNEVS July 14, 1959 B. G. HARPER ET AL 2,894,289

METHOD OF' MAKING PERMESELECTIVE MEMBRANES Filed March 1, 195e 2 sheets-sheet 2 l 1 O/o/f'ono/g, dry/'ng /he ,oef-mese/ec//ve I membrane produ-ef 1 INVENTORS. z //y Gene Har/oef .91 5 John Henry rown, Jr:

Jo/m C/or/fe Moore u l by United Parent' y O Tex., assignors to TheDow Chemical Company,.Mid`v land, Mich., a corporation of Delaware f Application March 1,1956, Serial No. 568,861

i` claims. (crisi-#57);

This invention relatesftoa process for producing porous membranes having selective permeability lfor various substances.

The specificdii'usivity potential for particular substances which may be exhibited by porous permeselective membranes, or those` possessing "characteristics of selective permeability, may "advantageously be utilized forv various purposeslincluding theseparation of` gaseous.'

materialsand vapors by fractional diiusi'on techniques.I The term, "permesel'ective' membrane,a`s he'rein employed, is intended 'to delineate those porous films and membranes which are essentially physically selective of lm, said selective solvent beingffsubstantially j;lieti-disl-lV solvent of at least one of the materials constituting said continuous phase iilm of theiilm-forming materialsffin the`iilm. Advantageously, polymeric materials may be employed as the nlm-forming materialsjin thefpractice" of the invention. It is usually beneficial; to utilize ra binary system of polymeric materials in` the preparationf of the permeselective membranes usingajcommbnsol-f vent Iwhich is capable of ltheir mutualfdissolultion and a" selective solvent which has no substantial or o'mplete4 dissolving `efficacy on one of4 the prziljiurreries`1r1ate'ri'als.`4

' In many cases, however, one or more of thelmaterials,

transfusing ory diilusingmaterials as distinguishedfrom y those which may be-'saidto function" more through chemical or electrical phenomena or effects In many instances,r.hlowever, the membranes whichordinarily have been available for `suchiapplications have -not provided completesatisfactionV andssuitable adaptability to warrant their Smore Widespread `commercial utilization and exploitation. Frequentlypforl example, the membranes mayf exhibitsuch ineliicient or impractically low `characteristics [of permeabilityand selectivity that inordinately highpressure dierentials or extremely large sur face areas, or. both, would be necessitated for the accomplishment `of` desirableI results and appreciable `productivity in diffusion processes,; .particularly in r gaseous` diffusionrprocesses. Furthermoreythe poor physical and chemicalstability under desired` conditions of employmentfor many ofthe known membranes precludes their being eiectively utilized 4for a Wide variety of materials which might be separable by various techniques, including gaseous, diiusiontechni'ques.l v

It is among the principal objects of thepresent invention to provide permeselective membranes from readily available materials that are imbued with high degrees of physical and-chemical stabilityand which have" `signiiicarnt and Aeiiicient characteristics of permeability and selectivity for various materials. It is also among the objects of the invention to provide foraminulate permeselective membranes `characterized in having finer andmore 'minutefacilities for physical transmission than mayy otherwise be obtained. It is among the additional objectives of the present invention to provide permeselective'membranes thatv are particularly adapted for employment in various diffusion processes, especially in gaseous diffusion processes. Other objects and advantages of the invention will be more apparent in the following description and specication.

1 Accordingto the invention, an ecient and stable permeselectivemembrane having exceptional diffusive selectivity for` various `substances may be obtained by dissolving at least two` dilerent materials in a common solvent, at least oneof said materials being alm-formling material, to provide a hlm-forming solution thereof;

forming a` continuousphase `film of said materials with said lm-forming solution; and extracting at least one of the `materials:.from*the lm with a selective solvent Afor. less than thefentire, portionriof the materials in the particularly the extractable 'mateiiaL-*nray fla" none" polymeric nature. The process of :thepresentfEinigen-fjV tion whereby the permeselective membranes nifay manufactured is schematically illustrated in `thema of a ow sheet diagram in Figure 5 of [the Vccornpanying drawing. Y

The permeselective membranes whichare' obtained,-4 as schematically illustrated in Figure? l fofthe accom-l panying drawing, are comprised of'a thin, usuallypliablel and iiexible film of polymeric orfother material having" a plurality of foraminulous pores or excee'dinglyqgdi minutive intersticial void spaces 'or passageways -in'its structure. This is further illustrated by the greatly magi niiied portion of a permeselective membrane shown in Figure 2 wherein its foraminulous character` may `be contrasted with the relatively solid iilm which is obtained'- prior to extraction as is illustrated, also in great magnification, in Figure 3. The foraminulous :passageways in the structure of the permeselective' membranesV-are vapors.

capable of diffusing various iiuid materials,)particularly gases and vapors, at independently transfu'sediratesfdef pending to a great extentuponthe respectivemo'lecul'ar Weights or'steric conligurations, or both; of` the vgases or The porosis of the permeselective membranes of the present invention, may, in many instances,.constitut`e' as much as 50 percent by volume in theslorm` offor-5 aminules or microforamina in andthrough: the intera sticed lm. A porosis` between about 35` and 40 perf cent by volume usually facilitates obtaining optimum diffusive and selective characteristics in` the membrane.' Suchiilms are especially well suited for employment as porous, selective barriers in diffusion processes,` including dialysis, osmosis and gaseous diffusion. Theyrcan be readily provided in a physically and chemically stable form, `depending upon the utilization of particular ilmforming materials which is made. They do not` require moisture-lor other forms of :planificationforwemployf mentl and may advantageously berused as astrong,`d,ry film structure.

A great variety of materials can be employed satisfactorily for obtaining permeselective membranes in ac;- cordance with the invention. In general, any combination of two or more materials, at least one of which is a lm-forming material, which are capable ofmutual employment and which are soluble to term ami'xed'lmformi-ng solution in a common solvent and Whichare susceptible to selective extraction by selective solvents to leave a membrane comprising porous nlm-forming material may be employed. It is frequentlydesirable, `as mentioned, to employ at least one polymeric lniforming material and it is usually beneficial to employ different materials to comprise the base membrane-wmf prising material and the extractablematerial. s

The ratio of materials which may be employed depends not only upon their solubility characteristics and the like in the particular solvent system which is utilized but also upon the characteristics desired inlth'e` permeselective membrane. Ordinarily, the porosis of membrane may be increased by utilizing greater proportions of the. extractable material in its-prepuaration' Whenthe i f Patented durifligformationofa solid. and. uniform lm, should be.

readilyremovable. by evaporation or other means, suchasby extractionwith another solvent which. has no. apprecibl'e effect upon the mixture of materials in thev film. The. selective. solvent should be capable of dissolving a large. portion. or substantially all of the extractable ma terials from the lm. to form the foraminulous membrane while leaving a stable and solid permeselective membrane consisting predominantly or substantially completely of the base materials; In some cases, asv` indicated, the selective solvent may bersuitably employed when it has` somesolvent elect upon the base material or materials which-remain to form the membrane, provided that, suchl eectisnot sucientto destroy the desirable stable and solidA microporous tilmstructure which is being prepared. In this` connection, the extraction of4 the extractable material from the llm need not be total or complete. by theV 'aeration process may be carried'. out under any desired conditions of temperature and pressure which may be suitable for their execution.` It is merely necessary to avoidv'conditions. under which the particular `materials being: processed might. tend to become unstable. The time; requirements will', lof' course, varyl with. the particular-materials. and conditions being employed for the preparation. of givenV membranes inthe conduction of the various steps of the process. The optimum conditions. of' preparation for various permeselective membranes with various` materialswill be `apparent rto those skilled'in the art upon consideration of -the several factor-s whichy may be` involved for the preparation of-"the given membrane. The membranes may be prepared to have 'anyl desired thickness to suit the anticipated conditions and'requirements of'their employment. It frequently-may ybe-benet`cifal` to rprepare membranes to have a thickness ofi at least` about 0.5 mil in order to provide them with suli'icient strength for practical use although, asy is apparent, this may vary with the ingredientmaterials and their respective proportions utilized for a given membrane as well as with the use for. which' it isV intended. Tlius, whether a membrane is lto be used for osmosis, dialysis or gaseous `dilTusion will also have a bearing on its optimum thickness.l

Although the invention is not intended to be restricted thereto, permeselective membranes having exceptional `characteristics for a wide variety of purposes are obtainable` when polymeric cellulose ethers such as ethyl cellulose are employed` as the base material and relatively low molecular weightpolyet-hylene glycols as the extractabe materials using chloroformas the common solvent for forming the film and water as the selective solvent: for the. polyethylenel glycol.

Thus,.rthe base material may `advantageously be comprised of `-an ethyl cellulose having, an ethoxyl content usually-designated standard, in the range from Iabout 478.70 to 49.5 percent by weight'which contains an average of' between aboutl 2.46.- and- 2.58- etheriied ethyl; sub'- stituentsper glucose unit and havingga viscosity less than about 30 centipoises, as determined in a solvent mixture consisting of about 'parts by volume of toluene and 20 parts by volume of ethanol. Such a material is available from the Dow Chemical Company under the trade-name EthoceL The extractable polyethylene glycol may have a molecular weight, for example, between about 1 `and 30 thousand andl may be employed with especial advantage in amounts between about one-third and one part by weight for each part by weight of the ethyl'. cellulose;

Gener-ally, ordinary temperatures and atmospheric pressures can be employed for the preparationA of' ethyl cellulose permeselective membranes with extractable polyethylene glycols. SuitableJ llm-forming `solutions of lthe mixed polymeric materials in chloroform can usually be preparedv within an hour or so at room temperature and under atmospheric pressure. Uniform l to 2 mil lms-can be cast. and evaporatedy to. dryness under the same conditions withinv time periodsL o several. hours. The polyethylene glycol can bereonveniently and thoroughly extracted with; the; formation' ofi an excellent membrane when the; selective. water solventis: used; ata.

temperature of about il C., for: a time. period. olthirty-v six hours lor so. Permeselective ethyl: 'cellulose membranes having; 25` to 50l percent porosis. can'- be easilyf prepared` in this. Way on when equivalent` conditionsfv are substitued. and. imposed in. the various processing: steps.. Permeselective: ethyl. cellulose; membranes mayv alsov beprepared by. employing a: non-polymeric extractabl'eA materialinthe.film-formingisolutio. soluble crystal;m line. inorganic:A materiali suchv as salt"` may.' thus be utilized: advantageously..

Cellulose nitratev membranes having` desirable char acteristics of selective-permeability may' also be prepared according to the4 inventionl wheni it is employedy as apolymeric Ibase material' withv otherv polyglycolV ma terials as. extractable. constituents. Thus, an extractable. material comprised of` a. water-soluble, relatively low molecular weight Z-ethylllrxyl", methyl diether ofapolyglycol consisting of heteric poly;(Ioxypropylenefoxy ethylene) glycol. whichhas abofutf'an ,equimolar proportion of.' oxypropylene and oxyethylene'- units in its molecule, may beA advantageously utilized with acellulose nitrate` consisting of cellulose triand tetra-nitrateas abase polymeric' material in a system wherein a mixture of ethylA ether and`v ethanol is employed 'as a 'common'- solvent andwater is utilized for selective extraction purposes.

The invention is further illustrated inand by the following examples.

Example l Av permeselective membrane. was prepared in, accordance with' the.. presenti. invention. and its.- performance as a selective barrier' for gases comparedwithconventionally obtainedv ethyl celluloselm, membranes. r[Phe lm, designated` E, wasprepared by-casting a chloroform solution of ethyl cellulose and'- ag polyethyleney glycol ma-y terial` ona glass plate and allowing the chloroform to evaporate and| leave'a solid, uniform residulum ilm structure ofthe mixed.- polymeric materials.` The'. ethylv cclluf lose employed was thestandardgradehaving aviscosity of about 10v centipoises; The polyethylene glycolA had a moleculary weight in the; neighborhood off about 4,000; The solution of the base and extractablepolymeric, materials containedabout 4.5 percent. -byiweight ofv ethyl cellulose and 2.0. percent by Weight of polyethylene gly. col,l based on. thetot-al weightA of the' solution, dissolved in the chloroform. The.v resulting` relatively' strong; and stable film, havin-ga thicknessof; about l; mil, was imirnersed in an'- ice water bath for about 36 hoursto extract `the polyethylene; glycol: and formftherpermeselective membrane; Theflm` F was thenfdried` frfuse.'y

The lilm. E was then; tested :to: determinel its; permeability constants; zfor substantially; hydrogen: and carasini-asa3 bon dioxide gases under varying pressure` differentials' across the film. The permeability constant is a representative measure of the number of cubic centimeters of gas under standard conditions of C. and one atmosphere of pressure that is passing through each square centimeter of area under a partial pressure across the membrane of one centimeter of mercury in a unit time of one second with a membrane having a film thickness of one centimeter. For purposes of comparison, similar tests were performed on a commercially available sheet of ethyl cellulose film, designated as film E, having a thickness' of about 5 mils, which was employed as a membrane. The results are graphically depicted in Figure 4 of the `accompanying drawing wherein `the permeability constants of the film for the gases are plotted against an applied pressure differential across the film to form the curves which are shown. It was observed that the diffusion rate of carbon dioxide through film E was about 1.5 times as great as the diffusion rate of hydrogen through the same hn excepting in the areas of very sli-ght pressure diiferential. However, while the diffusion rate of canbon dioxide was only slightly greater through film F than through film E, the film F was adapted to pass hydrogen more rapidly than the film E -by a factor of about 5.

In another test, not illustrated in the graph, another film of ethyl cellulose was prepared in a manner similar to that employed for film F excepting that the film casting solution contained only about 4.5 percent of the cellulose ether in chloroform. This iilm, upon testing, displayed essentially the same permeability characteristics as lm E.

Example I1 Permeselective membranes in the form of sheets were prepared from ethyl cellulose in a manner similar to that which is set forth in Example I. The sheets were tested for their performance as dialysis membranes by being placed in suitable apparatus in such a manner that an ionic solution could be passed over one surface of the membrane in the apparatus while distilled water was owing over the opposite side of the membrane. The rate of ionic diffusion through the membrane could be determined by testing samples of the distilled water for ion content. For purposes of comparison, a sample of commercial ethyl cellulose film, similar to that employed in the iirst example, was similarly tested. The rate of hydrogen and choride diffusion from a 0.1 N solution of hydrogen chloride in water through the permeselective membrane to the distilled water was found to be about 0.00002 moles per minute per square centimeter of the membrane per 0.001 inch of its thickness. Ihere was no evidence of any diffusion through the commercially available ethyl cellulose film.

Example III A permeselective membrane was prepared in the form of a tube (having a closed end) from ethyl cellulose, using a filmaforming solution similar to that described in the first example and a technique adapted therefrom. The tube h-ad a diameter of about 1 inch and a height of about inches. It was filled with a saline solution and immersed to a depth of about 3 inches in a Ibath of distilled water. Within a short period of time after its immersion, the level of the liquid inside the tube had risen about an inch higher than the level of the distilled water bath. Periodic testing of the liquid outside the tube disclosed that the salt was slowly diffusing into the distilled water bath. During this process, in which there were required about 5 days for an equilibrium condition to develop, the liquid level inside the tube returned to an equal height with the level in the bath.

6 Example IV About 20 grams of theV same cellulose ether as was employed in Example I and 15 grams of calcium nitrate were dissolved in about 400 grams of ethanol. A layer of theresulting film-forming solution was spread on a clean ,glass plate and dried to a uniform white film in several hours. Theiilm, remainingattached to the glass, was placed in a water bath for a several hourpe-` riod during which time it assumed agrey coloration.' The film was` removed froml the water and lpermitted to dry. It was then tested in the manner described in Example I for permeability to hydrogen gas. The rate of hydrogen diifusionthrough the film was found to be about times as great as that ofV the commercially available ethyl cellulose film tested in the first example.

Similar results are obtainable with permeselective membranes prepared in accordance with the invention from ethyl cellulose (or other cellulose ethers) and a polyethylene glycol having a molecular weight of about 9,000; cellulose nitrate and a water-soluble 2-ethylhexyl methyl diether of a heteric poly-(oxypropylene-oxyethylene) glycol; and other desired combinations of various base and extractable materials including, for example, polyvinylpyrrolidone, polyacrylic acid, polyacrylamide, and the like, and other salts and nonfpolymeric extractable materials.

Certain changes and modifications can be readily entered into in the practice of the present invention without substantially departing from its intended spirit and scope. It is, therefore, to be fully understood that all of the foregoing description and specification is to be taken as being merely illustrative of certain embodiments of the invention which is not intended to be limited or restricted thereby. Rather, the invention is to he interpreted and construed in the light of what is set forth and defined in the hereto appended claims.

What is claimed is:

1. A process for the preparation of permeselective membranes which comprises dissolving at least two different materials, at least one of said materials being a filmforming material, in a common solvent to provide a filmforming solution thereof; depositing said solution in a lmiform layer; removing said common solvent from the filmiform layer of said film-forming solution to form a continuous phase film of said materials and extracting at least one of the materials from the lilrn with a selective solvent for less than all the materials in the film, said selective solvent being substantially non-dissolvent of the film-forming materials constituting said continuous phase film.

2. A process in accordance with the process set forth in claim 1 wherein at least one of the film-forming materials is a film-forming polymeric material.

3. A process in accordance with the process set forth in claim 1 wherein a binary system of polymeric materials is employed.

4. A process for the preparation of permeselective membranes which comprises dissolving a base, film-forming, polymeric material and an extractable material in a common solvent to provide a film-forming solution -thereof; depositing said solution in a iilmiform layer; removing said common solvent from said lmiform layer of said film-forming solution to form a film of said materials;

and extracting the extractable material from the film with a selective solvent therefore; said selective solvent being substantially non-dissolvent of said base polymeric material.

5. A process in accordance with the process set forth in claim 4 wherein the extractable material is a polymeric material.

6. A process in accordance with the process set forth in claim 4 and including, as a final step, the step of drying the extracted lm.

7. A process in accordance with the process set forth in claim 4 wherein the ibase vpolymeric material is ethyl cellulose. and lthe extractable material is a polymeric material comprising vapo1yethy1ene glycol.

-8. A process in accordance with the process set forth in claim 7 wherein between about 1 and 3 parts vby weight of the ethyl cellulose is employed for each part by weight of -the polyethylene glycol.

9. AA .process in accordance with the process set forth in c1am4 wherein the base Vpolymeric: material isfcellulose nitrate and the extractable material vis a .polymeric material comprising a relatively low molecular weight 2- ethyflhexyl, methyl diether of la polyglycol consisting of heteric poly(oxypropylene-oxyethylene) Iglycol.

10. A process in accordance with the process set forth 8 in vclaim 4 wherein :the `base polymeric material is ethylv cellulose Sandthe extractable material ,comprises an inorganic, crystalline material.

IReferences Cited inthe .i'le of fthis patent 1 UNITED STATES PATENTS 2,490,178 'Kroon Dec. 6, 1949 2,503,139 Soler Apr. 4, .1950 2,504,208 Locke et al. Apr. l18, 1950 2,542,527 Honey et al Feb. 20, 19511 2,563,644 Drisch et al. Aug. 7, 1951 2,700,694 Fernald Jan. 25, 1955 2,846,727 lBechtold Aug. 12, 1958 

1. A PROCESS FOR THE PREPARATION OF PERMESELECTIVE MEMBRANES WHICH COMPRISES DISSOLVING AT LEAST TWO DIFFERENT MATERIALS, AT LEAST ONE OF SAID MATERIALS BEING A FILMFORMING MATERIAL, IN A COMMON SOLVENT TO PROVIDE A FILMFORMING SOLUTION THEREOF; DEPOSITING SAID SOLUTION IN A FILMIFORM LAYER; REMOVING SAID COMMON SOLVENT FROM THE FILMIFORM LAYER OF SAID FILM-FORMING SOLUTION TO FORM A CONTINUOUS PHASE FILM OF SAID MATERIALS AND EXTRACTING AT LEAST ONE OF THE MATERIALS FROM THE FILM WITH A SELECTIVE SOLVENT FOR LESS THAN ALL THE MATERIALS IN THE FILM, SAID SELECTIVE SOLVENT BEING SUBSTANTIALLY NON-DISSOLVENT OF THE FILM-FORMING MATERIALS CONSTITUTING SAID CONTINUOUS PHASE FILM. 